Physical models of vascular systems have proven to be very useful tools for studying hemodynamic events that occur during the use of catheters. Our investigations with in vitro flow models have included (1) mixing studies in a hepatic model using a prototype balloon catheter that infused dye solutions through side holes proximal to the balloon at the rate of 1.4ml/min, (2) studies in a hepatic model that investigated the effect of infusion rates (in the range from lml/hour to 20 ml/min) on mixing from an end hole catheter placed in the gastroduodenal artery, (3) mixing studies in a carotid artery model during ultra-slow infusion (4ml/day) from an Infusaid pump, (4) investigation of a novel balloon catheter with a built-in ultrasonic, pulsed-Doppler crystal in the tip intended to monitor intra-lumenal flow rates, and (5) studies in large-vein flow models of the use of multilumen catheters during simultaneous infusions of incompatible drugs.